(i) Field of the Invention
The present invention relates to a paper web forming apparatus and more particularly to a paper web forming apparatus of the type employable, for instance, for a double wire type wire part which is designed to include a section where two wires are adapted to move in the distance from a supporting roll to the next supporting roll while raw material (stock) is held therebetween and a single wire type wire part which is so designed that two wires can move in parallel with one another with the aid of top wire arranged additionally.
(ii) Description of the Prior Art
Wire parts of FOURDRINIER paper machines are generally classified into two types, that is, single wire type and double wire type. This FOURDRINIER paper machine is so constructed that raw material including fiber slurry and filler at a predetermined consistency is caused to uniformly disperse in a head box and thus uniformly dispersed stock material is quickly transferred to a wire part at which drainage is achieved to produce, a product in the form of paper web (wet paper). It appears that in said machine basic formation of the paper web (flock recognizable due to non-uniformity in entangling and distributing of fibers in paper web) is dependent on performance of the head box and an effect on the wire part is recognized only to the foremost part thereof at which dewatering is initiated. In practice, all the conventional wire parts are designed on the basis of the above-mentioned concept. Thus, wire part components or the like disposed for the purpose of better formation (dispersion of fibers) can not be looked for in the practical machine.
To facilitate understanding of the present invention it will be helpful that the typical wire types will be described below with reference to FIGS. 4 to 11.
First, FIG. 4 is a schematic side view illustrating an example of the single wire type. In the drawing reference numeral 1 designates a head box from which uniformly dispersed raw material 2 is injected. Thus injected raw material is then transferred on the wire 4 which are supported by means of a forming board 3. During movement of raw material on the wire in the direction as identified by an arrow mark in the drawing raw material is subjected to dewatering with the aid of a plurality of foils 5, table rolls 6, vacuum foil boxes 8, suction boxes 9 and a suction couch roll 10. A deflector 7 serves to scrape off white water which drained by means of the table rolls 6, while the wire is held in the horizontal posture.
Consistency of raw material is normally determined in the range of 0.5 to 1% at the outlet of the head box 1 and it increases to a level of 3 to 6% when raw material reaches the suction box 9. After it moves past the suction couch roll 10, it is dehydrated to a level of about 20% whereby it becomes a wet paper web. The produced wet paper web is taken out from and change the wires 4 under the effect of suction which is generated by means of a suction pickup roll 11 and it is then transferred on a felt 12.
FIG. 5 is an enlarged side view of the foils 5 which illustrates change of pressure exerted on raw material held on the wire 4 by the foils 5 and FIG. 6 is an enlarged side view of the table rolls 6 which illustrates change of pressure exerted on raw material held on the wire 4 varies by table rolls 6.
As is apparent from FIG. 4, the vacuum foil box 8 is so constructed that a plurality of foils are arranged one after another on the upper surface of the box which is evacuated to a level of comparatively low vacuum whereby dewatering enhanced. The suction box 9 is so designed that a perforated plates with a number of elongated holes or round holes formed thereon in plural rows are placed on the upper surface of the box whereby forcible dewatering is achieved under a high intensity of vacuum atmosphere. Next, the suction couch roll 10 is so designed that a suction box is disposed inside the perforated cell whereby powerful dewatering is achieved.
FIG. 7 is a schematic side view of an example of the double wire type that is so called gap type and FIG. 8 is a fragmental side view of the apparatus in FIG. 7, shown in an enlarged scale. Raw material 22 is injected from the head box 21 and it is then caused to move upwardly while it is clamped between both of the wires 23 and 24. Dewatering is achieved under the effect of pressure which is generated by means of a plurality of forming shoes 25 in the presence of wire tension and white water which appears as a result of dewatering is removed by scraping operation of the forming shoes 25 on the inside of the apparatus as well as by centrifugal force on the outside of the same. Thus dewatered raw material is transferred to the felt via the suction box 26 having the curved perforated plates and the suction couch roll 27.
FIG. 9 is a schemtic side view illustrating another example of the gap system double wire type and FIG. 10 is a fragmental side view of the apparatus in FIG. 9, shown in an enlarged scale. In the illustrated example a plurality of dewatering blades 31 are alternately arranged on both the sides of the wires.
FIG. 11 is a schematic side view illustrating an example of the double wire type that is so called roll type. In the illustrated example raw material is dewatered under the effect of pressure which is generated by tension of the wires partially wound about the suction roll 27 as well as suction force which is generated by means of the suction roll. Incidentally, in some case the roll is designed in the solid structure. In addition to this various types are practically employed but basic concept is based on a combination of the above-mentioned types or its modification or improvement.
Description will be made below as to devices and components used for the wire part to have an effect on raw material (hereinafter referred to simply as components), excluding the forming board and the deflector. Naturally, each of the components is designed and constructed for the purpose of carrying out dewatering under the influence of vacuum or pressure in such a manner that their foremost end comes in contact with the wires with the exception of rolls.
Among the above-mentioned components the forming board 3, the table rolls 6 and the foils 5 are helpful for the purpose to improve formation of the paper web when they are used with reduced dewatering force, as long as consistency of raw material is not so rich compared with that in the head box (normally in the range of 0.5 to 1%). The manner of utilizing them is determined in accordance with the extent that the wire part for which they are in use exhibits its inherent performances and therefore it has very few effect on production of paper. With respect to the double wire type it is found that relatively good formation is achieved compared with the single wire type. However, even in the case of the double wire type the formation becomes deteriorated as consistency increases. For this reason, any practical paper making machine is operated under the operative condition relative to consistency of fibers not in excess of 1%.
The essential reason why no improved formation can be obtained by means of the wire part and thereby consistency of raw material in the head box can not be increased consists in function of dewatering as mentioned below which is carried out by means of the components supporting the wires while raw material moves together with the wires. As is well known, raw material held on the wire or between the two is subjected to dewatering as it moves, resulting in increased consistency and reduced fluidity. Raw material located in the area in the vicinity of the wire surface is increasingly concentrated under the influence of dewatering until a fiber mat is formed. Although the components serve to support the wire function to disperse the raw material, they fail to do so as fludity of raw material decreases. As a result, only little accumulation of dispersion effect can be expected with them.
Further, when function other than dewatering is imparted to raw material at the step where forming of fiber mat proceeds, there is a danger of damaging or injuring the fiber mat which is being formed. In practice, it is reported that the paper web having bad formation with grain-shaped flock extended over the whole area of the product is produced when the machine is operated under the condition of comparatively high consistency (in the range of 1 to 1.5%) in accordance with the double wire type while the components constituting the wire part are adjusted to exhibit a high intensity of function thereof. In this case such a pattern that thus produced fiber mat is torn in pieces is recognized with respect to the flock appearing on the fiber mat.
In view of the above-mentioned facts it is generally considered that characterizing features of formation dependent on the structure of the wire part to some extent but formation is fundamentally determined by performance of the head box without remarkable improvement in the area of the wire part.
To obviate the foregoing problem consistency of raw material particularly in the head box may be raised up to a higher level, for instance, in the range of 2 to 3%. This level of consistency of raw material corresponds to consistency of the same as measured at the position located before the suction box or that as measured at the position located in the proximity of the suction box, when the single wire type is employed for the machine. This means that dispersion function to be achieved for raw material in the area just before the suction box is dependent only on dispersion function which is achieved in the head box. However, since raw material having a higher consistency as mentioned above has a high level of viscosity compared with raw material having a consistency lower than 1% and moreover repeated flocking (representing such a state that fibers become granular due to an occurrence of entangling) lasts for a very short period of time (for instance, on the order of 1/100 second), it is difficult to develop a head box which can maintain dispersion of raw material properly until the latter is processed to a product in the form of paper and this is the reason why raw material having a higher level of concentration can not be used.